Soliton microcombs constitute chip-scale optical frequency combs, and have the potential to impact a myriad of applications from frequency synthesis and telecommunications to astronomy. The demonstration of soliton formation via self-injection locking of the pump laser to the microresonator has significantly relaxed the requirement on the external driving lasers. Yet to date, the nonlinear dynamics of this process has not been fully understood. Here, we develop an original theoretical model of the laser self-injection locking to a nonlinear microresonator, i.e., nonlinear self-injection locking, and construct state-of-the-art hybrid integrated soliton microcombs with electronically detectable repetition rate of 30 GHz and 35 GHz, consisting of a DFB laser butt-coupled to a silicon nitride microresonator chip. We reveal that the microresonator’s Kerr nonlinearity significantly modifies the laser diode behavior and the locking dynamics, forcing laser emission frequency to be red-detuned. A novel technique to study the soliton formation dynamics as well as the repetition rate evolution in real-time uncover non-trivial features of the soliton self-injection locking, including soliton generation at both directions of the diode current sweep. Our findings provide the guidelines to build electrically driven integrated microcomb devices that employ full control of the rich dynamics of laser self-injection locking, key for future deployment of microcombs for system applications.

Dual-comb interferometry is based on self-heterodyning two optical frequency combs, with corresponding mapping of the optical spectrum into the radio-frequency domain. The dual comb enables diverse applications, including metrology, fast high-precision spectroscopy with high signal-to-noise ratio, distance ranging, and coherent optical communications. However, current dual-frequency-comb systems are designed for research applications and typically rely on scientific equipment and bulky mode-locked lasers. Here we demonstrate a fully integrated power-efficient dual-microcomb source that is electrically driven and allows turnkey operation. Our implementation uses commercially available components, including distributed-feedback and Fabry-Perot laser diodes, and silicon-nitride photonic circuits with microresonators fabricated in commercial multiproject wafer runs. Our devices are therefore unique in terms of size, weight, power consumption, and cost. Laser-diode self-injection locking relaxes the requirements on microresonator spectral purity and $Q$ factor, so that we can generate soliton microcombs resilient to thermal frequency drift and with pump-to-comb sideband efficiency of up to 40% at mW power levels. We demonstrate down-conversion of the optical spectrum from 1400 to 1700 nm into the radio-frequency domain, which is valuable for fast wide-band Fourier spectroscopy, which was previously not available with chip-scale devices. Our findings pave the way for further integration of miniature microcomb-based sensors and devices for high-volume applications, thus opening up the prospect of innovative products that redefine the market of industrial and consumer mobile and wearable devices and sensors.

Laser diodes are widely used and play a crucial role in myriad modern applications including nonlinear optics and photonics. Here, we explore the four-wave mixing effect in a laser diode gain medium induced by the feedback from the high-Q microring resonator. This phenomenon can be observed at a laser frequency scan close to the microresonator eigenfrequency, prior to the transition of the laser diode from a free-running to a self-injection locking regime. The effect opens up the possibility for generation of remarkably low-noise, stable, and adjustable microwave signals. We provide a detailed numerical study of this phenomenon proven with experimental results and demonstrate the generation of the signals in the GHz range. The obtained results reveal the stability of such regime and disclose the parameter ranges enabling to achieve it. Cumulatively, our findings uncover, to our knowledge, a novel laser diode operation regime and pave the way for the creation of new types of chip-scale, low-noise microwave sources, which are highly demanded for diverse applications, including telecommunication, metrology, and sensing.

Optical frequency combs are a unique tool for fundamental metrology, spectroscopy, and a broad spectrum of applications. High-Q microresonators serve as a promising platform for the generation of coherent frequency combs. The key characteristics that determine the properties of an optical frequency comb are the microresonator dispersion parameters. We have developed and verified an original technique that allows one to measure the dispersion coefficients with a high accuracy for both crystalline and integrated microresonators with the free spectral ranges from gigahertz to terahertz. We demonstrate the generation of a soliton optical frequency comb in an integrated microresonator pumped by a laser diode in the self-injection locking regime at a wavelength of 1.55 μm and show that its spectral characteristics closely correspond to the measured microresonator dispersion parameters.

The advantages of high-quality-factor (high-Q) whispering gallery mode (WGM) microresonators can be applied to develop novel photonic devices for the mid-infrared (mid-IR) range. ZBLAN (glass based on heavy metal fluorides) is one of the most promising materials to be used for this purpose due to low optical losses in the mid-IR. We developed an original, to the best of our knowledge, fabrication method based on melting of commercially available ZBLAN-based optical fiber to produce high-Q ZBLAN microspheres with the diameters of 250 to 350 μ m. We effectively excited whispering gallery modes in these microspheres and demonstrated high quality factor both at 1.55 μ m and 2.64 μ m. Intrinsic quality factor at telecom wavelength was shown to be (5.4 ± 0.4) × 10⁸ which is defined by the material losses in ZBLAN. In the mid-IR at 2.64 μ m we demonstrated record quality factor in ZBLAN exceeding 10⁸ which is comparable to the highest values of the Q-factor among all materials in the mid-IR.